Picking up one entry for each category was no easy task because applications are divided between GTK and Qt, and there are programs that run in the console and do their job perfectly and are more lightweight than a GUI application. As such, I decided to leave terminal applications out of this review in the first place, but I provided a link below to an article which covers those. Applications listed below are built using various toolkits, not just a single one like GTK, however they are among the most lightweight graphical tools from their category.